I have a 90 gallon reef tank with 2 175 watt MH lights. The aquarium has been going for about 9 months, but I don't have any coralline algae growing yet, so I haven't ventured into trying corals yet.

My problem started when I didn't realize that my RO filter was bad. I ended up with a horrible algae bloom. I purchased a Phosphate test kit and realized that high phosphate levels were part of my problem. From there I used a phosphate sponge to bring the levels down to around 0.03ppm. I reduced the time of my lighting period to only a couple of hours a day. Then I purchased more snails and hermit crabs to try and clean up the mess. I also worked hard with a toothbrush per advise here.

Things were going very well and I started to slowly increase the lighting period. However, I soon realized once the period was about 9 hours that my creatures couldn't keep up with the algae growth. I went back in and rechecked the phosphate level and it had risen back up to around 0.5 ppm.
(To bring this back down I purchased some more of the phosphate sponge and this morning the level was back around 0.03. This is an expensive way to run and aquarium!).

My question is what caused this? I checked my RO water and it is perfectly clean, according to my testkit (salifert). My calcium level is around 440 ppm which I thought would cause calcium phosphate to precipitate out ( I thought I read this somewhere, maybe on the Kalkwasser container). Was the increase in phosphate simply coming from the decomposed algae? Also, why am I not seeing lots of corraline algae growth? I started seeing some, but it seemed to stop possibly around the time of the increase in phosphate.

I have a 90 gallon reef tank with 2 175 watt MH lights. The aquarium has been going for about 9 months, but I don't have any coralline algae growing yet, so I haven't ventured into trying corals yet.

That is weird. In a 9 month period you should see a lot of coralline algae. For the two months my tank has been set up, it has totally covered the rocks. What is your Alk? With that high of Ca it should be taking off.

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My question is what caused this?

There are three ways that phosphate can be getting into your system: topoff/water changes, food, and salt mix.

Now you say that your RO water is now fine so I guess you either changed the membrane or got a new one, correct? If this is so, than either your food or salt mix is suspect. Make sure whatever salt mix you use says "Phosphate Free" on the package.

As for the food, what do you feed and how much? Frozen food is notoriously high in phosphates.

What are your nitrates, btw. I am not so sure that phosphates are only to blame there. Might be a compounded problem.

I would like to know the answer to this also,,, I will say what I have found which can be taken with a grain of salt...

I have PO4 problems around the 0.3 phase...
I do water changes 25% 4 times in 2 weeks. Level drops to almost non detectable...Then I test a few days later and they are about 0.2 or so...now...

I think the PO4 leaches out of the sand bed? it is rather strange... The water I use is RO and there are no detectable PO4 found in make up or change water...It has baffled me.. I feed every other day and no more than the fish and corals consume and NORI for the Tang.... Now, I know blender mush can contain PO4 because of the raw seafood used, however Nitrate I would think would be more evident... Not the case though.. Wierd Science

Phosphates inhibit coralline growth plus it also takes a tank some time to age.

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I didn't realize that my RO filter was bad

Do you have an RO unit or and RO/DI unit? RO does very little to phosphates, it's the DI filter that removes phosphates. On top of that fish foods and some additives contain phosphates.

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I checked my RO water and it is perfectly clean

Did you test freshwater with a saltwater kit? Most saltwater test kits are calibrated to a SG of 1.025 and if you used only fresh water that's why it didn't give you a reading.

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My calcium level is around 440 ppm which I thought would cause calcium phosphate to precipitate out

Calcium and alkalinity need to be in balance. If they are in balance and you have the proper amount of magnesium, then you won't end up with any snowstorms.

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it seemed to stop possibly around the time of the increase in phosphate.

Yes, as discussed above.

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Was the increase in phosphate simply coming from the decomposed algae?

Yes and no. The algae was masking the amount of phosphate really in your system. It wasn't in your water column to be tested for because the algae was grabbing as much of it as possible and using it for fertilizer. When you thought you had 0 phosphates, you really had tons, it was just bound up in the algae. (The same thing can happen with nitrates). So yes, you are correct that dieing algae is responsible for putting phosphates in the water column. However that's the best place for it to be as you can do a water change with RO/DI water (don't forget the DI) and start to remove this phosphate. You can buy addon DI units lots of places. In addition, using Kalk will cause phosphates to precipitate out of the water (but not out of living algae). The continued use of Kalk will help, adding DI will really help, and watching what types of foods you add to your tank will help.

Although, I hope you have test kits for Ph, Ca, and Alk. I don't recommend that people use Kalk unless they know all of these parameters.

Yes. (I have not gone back and verified his sources. However, he isn't a throw out estimates kind of guy).

Scary stuff indeed. That's why I like my refugium. I love Golden Pearls. Take a look at the list. The first week I used it I had a mess on my hands regarding phosphates. You learn to TARGET FEED corals with the stuff and not just throw it in the tank.